Saying this is perfectly possible, however, I am not certain that is what you want to say. Using the above question sounds like you are genuinely interested in what the one you are addressing thinks should be done in the case of you speaking German. Do you want to say something like "And how about I speak German?"? (Btw, another example of logical quoting, see the other topic...) In that case, you would phrase it along the lines of:じゃ(or さぁ)、ドイツ語でどう？or more politelyでは、ドイツ語ならどうですか。

Nah, I really mean it as a sort of rhetorical question. I imagine a lot of Japanese assume that all Westerners speak English, and so a situation like that can come up often. Why not assume they speak Finnish, or Dutch, or Italian instead? That was also rhetorical.